[MachineCopyPropagation] Handle propagation of undef copies

[critson]

Summary

When propagating undefined copies the undef flag must also be
propagated.

[foad]

What happens if you don't do this? Does it fail machine verification? I wonder why this bug hasn't come to light earlier?

[critson]

In D111219#3044995, @foad wrote:

What happens if you don't do this? Does it fail machine verification? I wonder why this bug hasn't come to light earlier?

Correct, it fails machine verification.
I suspect this is just a case of no one has generated MIR like this before.

[foad]

LGTM.

llvm/lib/CodeGen/MachineCopyPropagation.cpp

579–580

MOUse.setIsUndef(CopySrc.isUndef()) would be a bit neater, and the same for isRenamable. Dunno if it makes any practical difference.

[critson]

• Address reviewer comments

llvm/lib/CodeGen/MachineCopyPropagation.cpp

579–580

Sure.
Although doing that for isRenamable would definitely change output, so this change is probably not the right place for it.

llvm/lib/CodeGen/MachineCopyPropagation.cpp

for (unsigned OpIdx = 0, OpEnd = MI.getNumOperands(); OpIdx < OpEnd;
 
     LLVM_DEBUG(dbgs() << "MCP: Replacing " << printReg(MOUse.getReg(), TRI)
                       << "\n     with " << printReg(CopySrcReg, TRI)
                       << "\n     in " << MI << "     from " << *Copy);
 
     MOUse.setReg(CopySrcReg);
     if (!CopySrc.isRenamable())
       MOUse.setIsRenamable(false);
+    MOUse.setIsUndef(CopySrc.isUndef());
 
     LLVM_DEBUG(dbgs() << "MCP: After replacement: " << MI << "\n");
 
     // Clear kill markers that may have been invalidated.
     for (MachineInstr &KMI :
          make_range(Copy->getIterator(), std::next(MI.getIterator())))
       KMI.clearRegisterKills(CopySrcReg, TRI);
 
     ++NumCopyForwards;

llvm/test/CodeGen/AMDGPU/undef-copy-propagation.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -o - %s -march=amdgcn -mcpu=gfx1010 -run-pass=machine-cp -verify-machineinstrs | FileCheck %s
+
+---
+name: undef_copy_propagation
+tracksRegLiveness: true
+body:    |
+  bb.0:
+
+    ; CHECK-LABEL: name: undef_copy_propagation
+    ; CHECK: renamable $vgpr0 = COPY undef renamable $sgpr42, implicit $exec
+    ; CHECK-NEXT: renamable $vgpr1 = COPY undef $sgpr42
+    ; CHECK-NEXT: renamable $vgpr2 = COPY undef $sgpr42
+    ; CHECK-NEXT: renamable $vgpr3 = COPY undef $sgpr42
+    ; CHECK-NEXT: EXP_DONE 0, killed $vgpr0, killed $vgpr1, killed $vgpr2, killed $vgpr3, -1, -1, 15, implicit $exec
+    renamable $vgpr0 = COPY undef renamable $sgpr42, implicit $exec
+    renamable $vgpr1 = COPY renamable $vgpr0
+    renamable $vgpr2 = COPY renamable $vgpr0
+    renamable $vgpr3 = COPY renamable $vgpr0
+
+    EXP_DONE 0, killed $vgpr0, killed $vgpr1, killed $vgpr2, killed $vgpr3, -1, -1, 15, implicit $exec
+...